In this paper, considering variant power system parameters and using Imperialist Competitive Algorithm (ICA) and ITAE (Integral Time Absolute Error) criterion we deal with tuning optimal parameter of load frequency PID controller in two-area power systems. To attain the desirable robust performance, selecting the appropriate objective function is important. The obtained simulation results indicate that despite frequency deviation and Area Control Error (ACE), applying ITAE criterion leads to attain optimal control of parameters for power system using ICA, which has high accuracy and convergence speed. Moreover, performance of the proposed method is illustrated on a two-area power system under load disturbance. The obtained results indicate that the proposed optimization method has a desired and robust performance in a wide range of system parameters and load variations.